Machine-read codes are known in many variations in the state of the art, illustratively employing magnetic coded array, infrared-read coded array or the like. The main advantage of such codings or code-arrays is their security relative to forgery or deception. Moreover they are desired to be easily manufactured. Illustratively such coded arrays are used on identity cards, for instance automatic-teller cards, also on goods of all sorts which must be identified by means of apposed coded arrays, for instance by the code-reader of a department store cash register.
Apparatus of the initially cited kind that operate in an electrically capacitive manner are characterized by the ease with which an electrically conductive coded array may be provided. It may be bonded in the form of a metal foil or it may be printed on the substrate. The action being capacitive, the coded array can be coated with a covering film and be made invisible to the eye. The electrical method offers the advantage over the magnetic or optical ones that the reader apparatus is simpler, being required only to comprise electrical components. Complex magnetic or optical sensors are or may be eliminated.
Such apparatus is known from the German Offenlegungsschrift 22 52 046. However, this design incurs substantial drawbacks. It specifies the capacitances of the coupling sites. For that purpose the electrodes first must be moved above the coupling sites with accurate registry, namely precisely into the specified position. Next, the coded array must be stationary relative to the reader. Then the reader's voltage source must generate a single voltage jump. Thereupon the detector of the reader determines the charging current into the capacitors formed by the coupling sites and electrodes.
This design is quite susceptible to problems resulting, for instance from erroneous mechanical adjustments. The detector in the reader must be highly sensitive because it is required to display a single, quite weak charging current. Such a detector consequently is highly susceptible to external factors. The known apparatus requires one pair of electrodes for each pair of coupling sites. If the code should be lengthy, the reader must be very complex.